THE DROSOPHILA TGF-ALPHA HOMOLOG SPITZ ACTS IN PHOTORECEPTOR RECRUITMENT IN THE DEVELOPING RETINA

In vertebrates and Drosophila, the Epidermal Growth Factor Receptor (E GFR) signal transduction pathway is important in the regulation of cel lular development. EGFR is bound by several activating ligands includi ng Transforming Growth Factor-a in vertebrates, and its homolog Spitz in Drosophila. It has been shown that Spitz and EGFR act in the develo pment of the Drosophila central nervous system and compound eye. Here we show that spitz function is required in developing ommatidia for th e first cell recruitment step, and that Spitz pro-protein is expressed in the retinal neurons as they begin to differentiate. We propose a ' two-key' model for additive signal transduction from EGFR and other re ceptor tyrosine kinases, via the Ras pathway, in the developing eye.